Hydrodynamics to limit boiler fouling

ABSTRACT

Methods and systems relate to generating steam from water that may be recycled in thermal oil recovery processes and is heated in tubes having non-obtrusive features to limit fouling formation. The tubes may include jets to generate enhanced flow mixing along an inner wall of the tubes in order to increase heat transfer and disrupt bubble nucleation. Employing the tubes with the inner wall having an average surface roughness of less than one micron may further facilitate disruption of the bubble nucleation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a non-provisional application which claims benefit under 35 USC §119(e) to U.S. Provisional Application Ser. No. 61/766,463 filed 19 Feb. 2013, entitled “HYDRODYNAMICS TO LIMIT BOILER FOULING,” which is incorporated herein in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

None.

FIELD OF THE INVENTION

Embodiments of the invention relate to methods and systems for generating steam from water that may be recycled in thermal oil recovery processes and is heated in tubes having non-obtrusive features to disrupt fouling formation.

BACKGROUND OF THE INVENTION

Several techniques utilized to recover hydrocarbons in the form of bitumen from oil sands rely on generated steam to heat and lower viscosity of the hydrocarbons when the steam is injected into the oil sands. One common approach for this type of recovery includes steam assisted gravity drainage (SAGD). The hydrocarbons once heated become mobile enough for production along with the condensed steam, which is then recovered and recycled.

Costs associated with building a complex, large, sophisticated facility to process water and generate steam contributes to economic challenges of oil sands production operations. Once through steam generators (OTSGs) often produce the steam. Even with extensive water treatment, fouling issues persist and are primarily dealt with through regular pigging of boiler tubes, which increases operating costs and results in a loss of steam production capacity that translates to an equivalent reduction in production.

Other steam generation applications that utilize cleaner water and do not depend on such pigging may include rifling along the tubes to provide flow mixing. Without these perturbations, forced circulation comes from pumps supplying the water to the tubes. These other applications may further utilize recirculation to make dry steam.

Therefore, a need exists for methods and systems for generating steam that enable efficient hydrocarbon recovery from a formation.

BRIEF SUMMARY OF THE DISCLOSURE

In one embodiment, a method of generating steam for oil production includes passing a first quantity of water through a tube of a boiler. Heat transfer across the tube increases temperature of the water in the tube to a temperature for boiling nucleation. Injecting a second quantity of the water jetted at multiple locations through a wall of the tube enhances flow mixing inside the tube along where the heating of the water occurs.

According to one embodiment, a steam generating system for oil production includes an economizer section having a tube through which water is passed from a first pump and heated by heat exchange across the tube with flue gases. Multiple jets disposed along the tube direct water into the tube from a second pump operated at a higher pressure than the first pump to enhance flow mixing inside the tube. A radiant section couples to receive the water heated in the economizer section and further increase temperature of the water to produce steam.

For one embodiment, a method of generating steam for oil production includes increasing temperature of water flowing through a tube in a first section of a steam generator by heat transfer from flue gases across the tube. The method utilizes at least one fouling mitigation feature during the increasing of the temperature selected from the tube having an internal average surface roughness less than 1.0 micron and injecting additional water at multiple locations into the water flowing through the tube to enhance flow mixing. Vaporizing at least some of the water that is preheated in the first section of the steam generator produces the steam in a second section of the steam generator by radiant heat transfer from combustion of a fuel into the flue gases.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention and benefits thereof may be acquired by referring to the follow description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic of a steam generating system for oil recovery and having auxiliary water mixing jets to limit fouling, according to one embodiment of the invention.

FIG. 2 is a schematic cross-section, along a tube length, of the jets disposed along a tube of the steam generating system for inline feed water convergence, according to one embodiment of the invention.

FIG. 3 is a schematic cross-section, transverse to a tube length, of an alternative tangential jet into a steam generator tube for creating a swirling flow disturbance inside the tube, according to one embodiment of the invention.

DETAILED DESCRIPTION

Embodiments of the invention relate to methods and systems for generating steam from water that may be recycled in thermal oil recovery processes and is heated in tubes having non-obtrusive features to limit fouling formation. The tubes may include jets to enhance flow mixing along an inner wall of the tubes in order to increase heat transfer and disrupt bubble nucleation. Employing the tubes with the inner wall having an average surface roughness of less than one micron may further facilitate disruption of the bubble nucleation.

Turning now to the detailed description of the preferred arrangement or arrangements of the present invention, it should be understood that the inventive features and concepts may be manifested in other arrangements and that the scope of the invention is not limited to the embodiments described or illustrated. The scope of the invention is intended only to be limited by the scope of the claims that follow.

FIG. 1 illustrates an exemplary system that includes an injection well 100, a production well 101, a primary pump 102 and a boiler 104, such as a once through steam generator. While illustrated in an exemplary SAGD configuration, other techniques, such as cyclic steam stimulation, solvent assisted SAGD, steam drive or huff and puff, may employ the steam generated as described herein. The injection well 100 extends in a horizontal direction and above the production well 101 also extending in the horizontal direction.

In operation, steam enters the formation along the injection well 100 forming a steam chamber with heat transferred from the steam to the oil or bitumen in the formation. The oil once heated becomes less viscous and mobile enough for flowing by gravity along with condensate of the steam to the production well 101. A mixture of the condensate and oil collected in the production well 101 flows to surface where the oil to be sold is separated from the condensate, which is treated and recycled for generating additional steam to sustain steam injection.

With respect to generating the steam, the primary pump 102 supplies a primary feed stream 103 of the water, which includes the condensate, to the boiler 104. The boiler 104 includes an economizer section 106 and a radiant section 108. The economizer section 106 preheats the water by heat exchange with flue gases of the boiler 104 across a tube (206 shown in FIG. 2) through which the water flows.

Combustion of fuel, such as natural gas, fires the boiler 104 and produces the flue gases. The radiant section 108 further increases temperature of the water from the economizer section 106 by radiant heating from the combustion. In some embodiments, the water fed to the boiler 104 passes through the economizer and radiant sections 106, 108 once without recirculation.

To facilitate handling of impurities in the water, the boiler 104 may produce wet steam having a quality between 60 and 95 percent following such single pass. Separation of liquids from vapors output from the boiler 104 may provide 100 percent quality steam for introducing into the injection well 100. The liquids from such separation contain concentrated levels of the impurities and thus may require disposal or further treatment before reuse.

Without being limited to any particular theories, a majority of prior fouling issues may occur in a nucleate boiling regime such that disruption of bubble nucleation may facilitate prevention of the fouling. Bubbles form on walls of the tubes in the nucleate boiling region and collapse towards a center of the tubes since average temperature of the water in the tubes is below a boiling regime. The fouling may initiate by physics of the bubble generating mechanism and be exacerbated because this nucleate boiling regime has higher wall temperature than where the water remains sub-cooled and lower flow velocity than where the boiling regime is developed.

Changes in concentrations of dissolved species in the water occur during the bubble generation as more volatile species enter the vapor phase and concentration gradients form around the bubble. Phase instability may occur if the local concentrations of hydrocarbons and/or phenols outside or inside of the bubble exceed their equilibrium limits, resulting in the fouling. In addition, any increase in the wall temperature may drive more polymerization reactions of hydrocarbons, which may be concentrated near the wall due to the concentration gradients relative to a remainder of the mixture and provide the fouling once polymerized into longer chains.

Any insoluble products may deposit on the walls of the tubes as the bubbles detach from the walls of the tubes. Relative slower velocity of the flow in the tubes towards the wall of the tubes further promotes the fouling since likelihood of deposits becoming entrained decreases as the velocity reduces. Reducing the time scale for bubble release or disrupting bubble growth may also inhibit the fouling since such fouling generation has an implicit requirement that reaction time required for the polymerization must be less than the time for bubble generation and detachment.

In operation, initial steam generation and hence the boiling nucleation regime occurs in the economizer section 106, which may include one or more fouling mitigation features as described further herein and that may likewise be employed in the radiant section 108. In some embodiments, a secondary pump 110 pressurizes part of the water to provide a secondary feed stream 111 at a pressure higher than the primary feed stream 103. First jets 112 and/or second jets 114 introduce the secondary feed stream 111 into the primary feed stream 103 respectively along the economizer section 106 and/or the radiant section 108.

FIG. 2 shows the jets 112 disposed at spaced apart locations along a length of the tube 206. With respect to aforementioned fouling mechanisms, the jets 112 may function to accelerate a boundary layer and increase heat transfer rate in the tube 206. Further, added flow mixing next to the wall of the tube 206 facilitates in disruption of the bubble nucleation to limit polymerization of hydrocarbon compounds.

For some embodiments, the tube 206 includes at least two jets 112 spaced from one another along the length of the tube 206 with number and frequency of the jets 112 depending on how long flow in the tube 206 takes to return to a steady flow state. Further, the jets 112 may introduce the secondary feed stream 111 at different circumferential locations dispersed from one another by being disposed around a circumference of the tube 206. FIG. 2 depicts such an exemplary placement of the jets 112 on top and bottom of the tube 206 at each of two locations along the tube.

The jets 112 may define a flow path port for the secondary feed stream 111 into the tube 206 aligned at an acute angle with flow of the primary feed stream 103. Acuteness of the angle helps ensure that the secondary feed stream 111 generates enhanced mixing along the boundary layer. Such mixing relies on relative high momentum and low mass flow rate of the secondary feed stream 111 compared to the primary feed stream 103.

One implementation of the jets 112 ensures that the jets 112 do not introduce any restrictions to an inside diameter of the tubes 206. While the fouling may be reduced, periodic pigging of the tubes 206 may still occur and thus not be obstructed by the jets 112. Existing once through steam generator designs and operations may implement the jets 112 by: tapping a hole in the wall of the tube 206; threading an end cap into the hole; drilling a diagonal slot on a side of the end cap into the tube 206; tapping over the diagonal slot with a larger tap than a diameter of the slot; and connecting a supply line, including any nozzle assembly, for the secondary feed stream 111 to the tap in the end cap.

In some embodiments, the fouling mitigation feature used includes polishing or treatment of an inner wall surface 207 of the tube 206. For some embodiments, roughness averages for the inner wall surface 207 of the tube 206 range from 0.2 to 0.1 microns or less than 1.0 micron compared to conventional hot worked tubing that has a roughness average of 12.5 microns. Grinding or acid treating an inside of the tube 206 to at least reduce cavities or coating the inside of the tube 206 with a material, such as ceramic, may provide this desired finish for the inner wall surface 207.

Vapor bubbles initiate in the cavities (i.e., rough surfaces) of the inner wall surface 207 of the tube 206 during the nucleate boiling. Thus, level of vapor formation on the inner wall surface 207 of the tube 206 decreases with lower surface roughness of the inner wall surface 207 of the tube 206. In addition, lowering the contact angle with water and hence making the inner wall surface 207 of the tube 206 more hydrophilic tends to inhibit vapor bubble formation.

Increasing the superheat necessary to generate the vapor bubbles due to selection of the inner wall surface 207 of the tube 206 hence decreases the bubble size and the hydrocarbon concentration gradients. As discussed herein, such factors may facilitate in mitigating the fouling. This treatment, polishing or coating of the inner wall surface 207 of the tube 206 may benefit the fouling problem while reducing a heat transfer coefficient, which impact may be more acceptable in the economizer section 106 than in the radiant section 108 that has a higher heating rate.

FIG. 3 illustrates an alternative tangential jet 312 into a steam generator tube 306 for creating a swirling flow disturbance inside the tube 306. Flow of a secondary feed 311 of water through the tangential jet 312 and into a primary water flow in the tube 306 occurs tangential to the primary water flow. The tangential jet 312 thus defines a flow path port for the secondary feed stream 311 at an acute angle into the tube 306 and perpendicular with the primary water flow.

Similar to the jets 112 shown in FIG. 2, the swirling flow disturbance caused by the tangential jet 312 increases heat transfer at a wall of the tube 306 and disrupts bubble growth such that there is limited time for polymerization chemical reactions to occur. Given the swirling flow disturbance, applications may rely on only the tangential jet 312 at one location around the tube 306 per longitudinal interval without need for multiple circumferential placements in such longitudinal interval to achieve desired flow mixing in the boundary layer. Utilizing the tangential jet 312 at multiple discrete spaced locations along the length of the tube 306 maintains desired flow properties in the tube 306.

For some embodiments, pulsing flow or fluctuating fluid pressure supplied to the jets 112 or the tangential jet 312 facilitates cleaning away of fouling build up. In some embodiments, fluid supplied to the jets 112 or the tangential jet 312 includes a fouling inhibitor chemical agent. Examples of such agents include additives known or used in the past to mix with the water before heating with added benefit of this delivery approach enabling a limited quantity of agent required due to local application of the agent along where fouling occurs and/or efficient mixing of the agent in the water.

According to some embodiments, the fluid supplied to the jets 112 or the tangential jet 312 includes a polishing material to remove fouling build up. The polishing material thus may include a solid particulate or abrasive added to the water and directed to where the fouling occurs. A blow down section downstream after the heating may enable removal of the polishing materials and any solids cleaned out by the polishing material.

In closing, it should be noted that the discussion of any reference is not an admission that it is prior art to the present invention, especially any reference that may have a publication date after the priority date of this application. At the same time, each and every claim below is hereby incorporated into this detailed description or specification as additional embodiments of the present invention.

Although the systems and processes described herein have been described in detail, it should be understood that various changes, substitutions, and alterations can be made without departing from the spirit and scope of the invention as defined by the following claims. Those skilled in the art may be able to study the preferred embodiments and identify other ways to practice the invention that are not exactly as described herein. It is the intent of the inventors that variations and equivalents of the invention are within the scope of the claims, while the description, abstract and drawings are not to be used to limit the scope of the invention. The invention is specifically intended to be as broad as the claims below and their equivalents. 

1. A method of generating steam for oil production, comprising: passing a first quantity of water through a tube of a boiler; heating the water in the tube by heat transfer across the tube to a temperature for boiling nucleation; and injecting a second quantity of the water jetted at multiple locations through a wall of the tube to induce enhanced flow mixing inside the tube along where the heating of the water occurs.
 2. The method according to claim 1, wherein the second quantity of the water is jetted aligned with flow through a length of the tube.
 3. The method according to claim 1, wherein the multiple locations are spaced along a length of the tube.
 4. The method according to claim 1, wherein the multiple locations are spaced along a length of the tube and are circumferentially dispersed around the tube.
 5. The method according to claim 1, wherein the second quantity of the water is jetted tangential to flow through a length of the tube.
 6. The method according to claim 1, wherein the second quantity of the water is injected into the tube without restricting an internal diameter of the tube.
 7. The method according to claim 1, further comprising polishing an inside of the tube to have an average surface roughness between 0.2 to 0.1 microns.
 8. The method according to claim 1, further comprising one of acid treating and coating an inside of the tube to provide an internal average surface roughness less than 1.0 micron.
 9. The method according to claim 1, further comprising fluctuating pressure of the second quantity of the water injected through the wall to contribute to the flow mixing.
 10. The method according to claim 1, further comprising adding at least one of a fouling inhibitor chemical agent and a polishing material to the second quantity of the water injected through the wall of the tube.
 11. The method according to claim 1, further comprising injecting steam produced from the water that is heated into an injection well and recovering condensate that is recycled to resupply the water to the tube.
 12. The method according to claim 1, wherein the heating occurs in an economizer section of a steam generator to increase temperature of the water that then passes to a radiant section of the steam generator to produce the steam.
 13. A steam generating system for oil production, comprising: an economizer section having a tube through which water is passed from a first pump and heated by heat exchange across the tube with flue gases; multiple jets disposed along the tube to direct water into the tube from a second pump operated at a higher pressure than the first pump to create enhanced flow mixing inside the tube; and a radiant section coupled to receive the water heated in the economizer section and further increase temperature of the water to produce steam.
 14. The system according to claim 13, wherein the jets are spaced along a length of the tube and are circumferentially dispersed around the tube.
 15. The system according to claim 13, wherein the jets are disposed to introduce the water from the second pump tangential to flow through a length of the tube.
 16. The system according to claim 13, wherein the tube has an internal average surface roughness less than 1.0 micron.
 17. The system according to claim 13, further comprising an injection well coupled with an output of the radiant section for conveying the steam into a formation to facilitate the oil production.
 18. A method of generating steam for oil production, comprising: increasing temperature of water flowing through a tube in a first section of a steam generator by heat transfer from flue gases across the tube, wherein at least one fouling mitigation feature is utilized during the increasing of the temperature and is selected from the tube having an internal average surface roughness less than 1.0 micron and injecting additional water at multiple locations into the water flowing through the tube to create enhanced flow mixing; and vaporizing at least some of the water that is preheated in the first section of the steam generator to produce the steam in a second section of the steam generator by radiant heat transfer from combustion of a fuel into the flue gases.
 19. The method according to claim 18, further comprising injecting the steam into an injection well and recovering condensate that is recycled to resupply the water to the steam generator.
 20. The method according to claim 18, further comprising injecting the steam into a horizontal injection well and resupplying the water to the steam generator from condensate recovered in a horizontal production well disposed below the injection well. 